add npm to install instructions in README.md
add npm to install instructions in README.md

Summary: Fixes https://github.com/facebook/flow/issues/2451

Reviewed By: jeffmo

Differential Revision: D3877782

fbshipit-source-id: 4ed018e3a5c452a517dde2201b9be3caf847f3e8

jgrund/flow,mroch/flow,fletcherw/flow,TiddoLangerak/flow,facebook/flow,fletcherw/flow,chenglou/flow,ForbesLindesay/flow,nmote/flow,samwgoldman/flow,nmote/flow,popham/flow,fletcherw/flow,popham/flow,AprilArcus/flow,mroch/flow,MichaelDeBoey/flow,nmote/flow,JonathanUsername/flow,jgrund/flow,ForbesLindesay/flow,AprilArcus/flow,ylu1317/flow,AprilArcus/flow,nmote/flow,AgentME/flow,JonathanUsername/flow,nmote/flow,MichaelDeBoey/flow,nmote/flow,AprilArcus/flow,JonathanUsername/flow,AgentME/flow,chenglou/flow,samwgoldman/flow,mroch/flow,AgentME/flow,gabelevi/flow,facebook/flow,JonathanUsername/flow,gabelevi/flow,samwgoldman/flow,jgrund/flow,jamesgpearce/flow,JonathanUsername/flow,samwgoldman/flow,AgentME/flow,gabelevi/flow,jgrund/flow,facebook/flow,gabro/flow,MichaelDeBoey/flow,mroch/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,jamesgpearce/flow,TiddoLangerak/flow,claudiopro/flow,chenglou/flow,gabro/flow,TiddoLangerak/flow,mroch/flow,chenglou/flow,ylu1317/flow,popham/flow,gabro/flow,gabelevi/flow,facebook/flow,popham/flow,gabro/flow,facebook/flow,chenglou/flow,mroch/flow,jamesgpearce/flow,gabro/flow,ylu1317/flow,MichaelDeBoey/flow,claudiopro/flow,jgrund/flow,popham/flow,ylu1317/flow,fletcherw/flow,facebook/flow,TiddoLangerak/flow,JonathanUsername/flow,fletcherw/flow,samwgoldman/flow,samwgoldman/flow,AgentME/flow,gabelevi/flow,ForbesLindesay/flow,JonathanUsername/flow,jgrund/flow,chenglou/flow,gabro/flow,nmote/flow,AprilArcus/flow,claudiopro/flow,ForbesLindesay/flow,claudiopro/flow,jamesgpearce/flow,TiddoLangerak/flow,ForbesLindesay/flow,fletcherw/flow,AprilArcus/flow,jamesgpearce/flow,mroch/flow,ylu1317/flow,jamesgpearce/flow,popham/flow,popham/flow,claudiopro/flow,MichaelDeBoey/flow,jamesgpearce/flow,ForbesLindesay/flow,gabelevi/flow